Down Syndrome (DS) is a neurodevelopmental disorder, affecting 1 in every 700 newborns in the US. DS patients exhibit impaired language ability and developmental delays in cognitive functions, including learning and memory. Though DS is caused by the presence of an extra copy of chromosome 21, how such genetic change leads to altered physical and neurocognitive growth remains largely unknown. Consequently, no therapies or drugs are currently available for impaired cognition associated with DS. A major obstacle to understanding DS has been the lack of model systems representing the development of human neural circuitry. Recent advances in induced pluripotent stem cells (iPSCs) technology have made it possible to investigate pathogenesis of DS in human cellular models. These elegant recent studies revealed altered neuronal morphology associated with DS, however, no significant physiological alterations in neuronal and synaptic activity were observed in neurons derived from iPSCs of DS patients. Astrocytes are an important and somewhat underappreciated neural cell. At synapses, astrocytes make contacts with pre- and post- synaptic neurons, acting as integrators and modulators of neural circuitry throughout the brain. Our preliminary results, as well as work of others indicate that astrocytes differentiated from DS patient-specific iPSCs adversely affect neurons. We therefore hypothesize that astrocytes play an important role in modulating neuronal activity in DS brains and that reconstruction of synapses formed between hiPSC-derived neurons and astrocytes may provide a window for the observation of neuronal phenotypes reflective of DS pathophysiology. We believe that a new culture system enabling analysis of neuron-astrocyte crosstalk with cell-type specificity and at the level of synapses will be better suited to study human neural circuitry than currently available culture and animal models. We propose to develop microfluidics-based cell culture system for modeling and analyzing the diseased neuron-astrocyte network of DS. The neural cells will be derived from patient-specific iPSCs and then organized into functional units of neural circuitry - tirpartite synapses - using microfluidics and surface micropatterning approaches. For sensing synaptic activity, we will implement genetically encoded intracellular biosensors to monitor the communication at synapses. This project will be an important step towards our understanding of DS pathophysiology by connecting genetic mutations associated with DS to the structure and function of the human neural circuitry.